Sounds a little more like a cult than a guild, although I suppose it could function sort of like an assassin's guild where it's not so much the death of the target that's wanted as the target's neutralization. Could be in service to some manner of organized crime as the boogymen to scare people into cooperating. I'm sure there are various plants and herbs that could easily cause insanity; I can't think of any off hand, but Wikipedia surely will have something. I know that there are certain drugs that can cause psychosis, schizophrenia-like symptoms, paranoia, etc.
Something you might want to consider (and I don't know how factually accurate this is), but I've heard stories of assorted traditions of young wives keeping elderly husbands in a drug-induced stupor and/or madness. They didn't want them dead (if the husband died, the wife was expected to follow suit), but by keeping them mentally incapacitated let them do as they pleased. So it would seem reasonable that this guild would be consulted if the target wasn't wanted dead.